1. Field of the Invention
The present invention relates to a charge-coupled device (CCD) type solid state image pickup device.
2. Description of the Related Art
CCD type solid state pickup devices instead of pickup tubes have been used in family video cameras, electronic news gathering (ENG) cameras and the like.
In a prior art CCD type solid state image pickup device including a semiconductor substrate having photo/electro conversion portions and a first insulating layer formed on the semiconductor substrate, a plurality of charge transfer electrodes are formed on the first insulating layer and are a single structure formed by polycrystalline silicon. Also, a second insulating layer is interposed between two adjacent ones of the charge transfer electrodes. This will be explained later in detail.
In the above-described prior art device, however, the parasitic resistance of the charge transfer electrodes is considerably large, due to the high resistance of polycrystalline silicon. Therefore, a time constant determined by the above-mentioned large parasitic resistance and a parasitic capacitance formed by the charge transfer electrode is increased which rounds the waveform of a voltage applied to the center of the charge transfer electrode, and reduces the charge transfer efficiency.
In order to reduce the parasitic resistance of the charge transfer electrodes, an optical shield layer can be connected to the charge transfer electrodes (see: K. Orihara et al., "New Shunt Wiring Technologies for High Performance HDTV CCD Image Sensors", IEDM92, pp. 105-108).
In this case, however, the optical shield effect is deteriorated which increases smear charges, which causes image defects. In addition, when the number of pixels is increased, the gap among the portions of the optical shield layer is also very small. For example, this gap is about 0.2 .mu.m. However, since the optical shield layer is made of tungsten or aluminum, not polycrystalline silicon, it is impossible to form such a 0.2 .mu.m gap in the optical shield layer.